Summary.A bioassay technique for the determination of phytotoxic residues of triazine herbicides and monuron in soil from field plots is described. The effect of a dilution series with untreated soil on shoot growth of Italian ryegrass (Lolium multiflorumLam.) is compared with that in a corresponding assay at the lime of application of the herbicide. This method was used to compare the persistence of residues of simazine, propazine, prometon and monuron, and to compare persistence in 3 years and in three soil types. The time required for disappearance of 80% of the activity following application of 2 lb/ac of simazine varied from 7 to 27 weeks, according to the year. Prometon was the most persistent of the triazines tested.Persistance de résidus phyloloxiques d'herbicides à base de triazines dans le s